In the gas turbine engine art, it has been found that certain components exposed in operation to relatively lower temperatures are conveniently constructed from fiber reinforced composite polymeric matrix (PMC) materials. The art has described a variety of methods for superimposing or stacking a plurality of reinforced, shaped layers, in a selected orientation, and then pressing and curing the layers into an article preform. Articles made in this way have included gas turbine engine blades, vanes and struts located in the forward or cooler-operating section of such an engine. For example, U.S. Pat. No. 3,892,612--Carlson et al., issued Jul. 1, 1975, describes one composite airfoil member and various aspects of making such a member. The disclosure of such patent is hereby incorporated herein by reference.
One evolution of such technology involved the development of composite airfoils, such as turbine engine fan blades and vanes, constructed of thin layers of fiber reinforced polymeric matrix material. For example, such fibers as carbon, graphite, glass, aramid, boron coated tungsten, etc. were impregnated with such polymeric materials. The layers were heat and pressure bonded together into an appropriate shape as an airfoil member.
During use of such a composite member, loads are transferred from one layer to the next by shear through joining material or interface. During an impact situation, as can occur from foreign, air-borne particles striking the airfoil, loads can bend the composite structure and generate large interlaminar shear forces which transfer loads from one fiber plane to the next. Since such composite structures are weaker in the "Z" axis direction (substantially normal to the lamination planes or zones), delamination can initiate.
It has been found that stitching together such composite structures in the "Z" direction can increase the structure's interlaminar properties. However, penetrating a needle carrying a thread or fiber-like member through such relatively thick, uncured materials can be difficult. It has been found that during such "Z" axis stitching using a needle, even driven using ultrasonic energy, the needle can act as a damper of such energy, and can weaken and break the notch sensitive "Z" axis stitching fiber while the needle and fiber are attempted to be driven through the laminated, uncured composite material.